The reason that the warship “Vasa” did not decompose is because there are no ship worms in the Baltic to eat the wood. This is because they live in low oxygen/very salty water
The lack of oxygen in the water there is a phenomenon that is well known for decades. The reason is that the Baltic is a sort of sack where water goes in from the Atlantic but can’t get out. It is saltier than the Atlantic too for the same reason
Which also makes it a bad choice as a sewage dump... I could see piping that stuff out to the open ocean.
“This is because they live in low oxygen/very salty water”
Correction, sentence should have read: This is because they CANNOT live in low oxygen / very high salt water
The Black Sea is even worse in this regard
One notable trait of the Baltic is that there are NO major rivers flowing into it.
As a consequence, your "sack" description above is explained: there is more flowage into the Baltic from the Atlantic than there is from the Baltic flowing out.